Excavations such as utility trenches, foundations and the like are generally backfilled in layers, termed lifts, with each lift of backfill material be compacted prior to the addition of the next lift. If backfill material is improperly compacted, or if an unsuitable backfill material is used, the backfilled excavation may subside. Subsidance in foundations, roadways or other structures due to improper backfill compaction can necessitate costly and disruptive repairs.
In the proper compaction of backfill in an excavation, each backfill lift should be properly compacted prior to the addition of the next lift; and after all lifts are in place and compacted, the complete backfilled excavation should be proofed to verify that there are no voids, soil bridges, or layers that are not fully compacted. Current monitoring methods such as drop hammer penetration probes, nuclear densitometry and the sand cone method are typically used only to check each lift for proper compaction, and such methods are expensive, time comsuming and cannot be performed in the real time.